Field of the Invention
The present invention relates to a polyimide (PI) having two —COOH capping groups at each end, a coating composition formed from the PI, and a flexible device using the coating composition.
Description of the Prior Art
A PI refers to a compound having a functional group of the following formula:

Since a PI has excellent thermal stability, and a film formed by a PI usually has good mechanical, electrical, and chemical properties, it is the first choice for high-performance polymeric materials.
In the semi-conductor industry, PI is widely applied in passive films, stress buffer films, α particle shielding films, dry etching protective covers, micro-electro-mechanical systems and interlayer insulating films. PI may also be used in electronic packages, enamel wires, printed circuit boards, sensing elements, and separating films. PI may also be used in thermosetting low-temperature hardening protective materials for active/passive devices, optical materials, touch panels, copper foil substrates, soft flexible electronic materials or integrated circuit elements.
Commonly seen PI can be classified as an uncyclized PI precursor, that is, a polyamic acid (PAA) PI, or a cyclized soluble polyimide (SPI). Currently, a reaction scheme for preparing the PAA PI may be briefly described as follows:

In the aforementioned preparation method, a diamine monomer is dissolved in a polar aprotic solvent, and then equimolar dianhydride monomer is added. Then, condensation is carried out so as to form a PI precursor, that is, PAA. Next, imidization of the PAA is carried out through heating, and after condensation, dehydration, and cyclization, the PAA is converted into the PI. The preparation method has a disadvantage that 100% imidization can only be achieved through heating over a long duration (generally, heating for several hours) at a high temperature of about 350° C. If the molecular weight of PAA obtained at the first stage does not reach a certain level (that is, the molecular weight is excessively low), a PI film having good physical properties cannot be obtained after imidization. However, if the molecular weight of the PAA obtained at the first stage is excessively high, the viscosity is too high, resulting in disadvantages such as poor operability and poor leveling property during coating. For example, when spin coating is performed, unleveling phenomena such as a convex center and thick edges would easily occur. Moreover, if the molecular weight of PAA is excessively high, when a second stage of imidization is performed, an extremely large internal stress is generated due to interaction between molecules and shortened bond length of a molecular chain, so that the coated substrate is bent and deformed.
Because the cyclized SPI has been imidized, it has a low thermosetting temperature (<270° C.) and excellent thermal stability. In order to improve the physical properties, the cyclized SPI usually has an active group at a capping site, so that thermosetting polymerization can be carried out. An example of the SPI is as follows:

However, during thermosetting of the SPI, a rigid structure is formed, and the SPI becomes brittle; moreover, after cross-linking, the cross-linking density is low, and the chemical resistance is poor.
Another example of the SPI is as follows:

The SPI has good reactivity when being heated. However, due to a structure problem of the SPI (i.e., it has a triple bond and an N═C═O group), during thermosetting of the SPI, a rigid structure is also formed, and the SPI becomes brittle and is prone to yellowing, thus resulting in a reliability problem.
The commonly arising conflict between material characteristics and operability makes it difficult for persons engaged in PI research to achieve a balance between these needs. Currently, there is still industry demand for a PI with improved physical properties (especially chemical resistance and excellent thermal properties, for example, a low CTE).